Water gas shift catalyst preparation and water gas shift process



Patented Mar. 10, 1953 WATER GAS SHIFT CATALYST PREPARA- TION AND WATERGAS SHIFT PROCESS Ottis W. Moak and William E. Spicer, Baton Rouge, La.,assignors to Standard Oil Development Company, a corporation of DelawareNo Drawing. Application October 27, 1949, Serial No. 123,995

11 Claims.

The present invention relates to the preparation of hydrogen by thereaction between a hydrocarbon, usually methane, and steam in thepresence of a reformer catalyst to produce mixtures of H2 and CO, andthe treatment of this gaseous mixture with further quantities of steamin the presence of a carbon monoxide converter catalyst. Morespecifically, the present invention relates to an improved process forthe preparation of a carbon monoxide converter catalyst wherein isobtained a catalyst having a more rugged nature and a higher activitythan those hitherto produced.

In the manufacture of hydrogen by the reaction of steam withhydrocarbons at elevated temperatures, it is well known to carry out theprocess in two stages, wherein in the first stage, methane or othergaseous hydrocarbons are reacted with preferably an excess of steam inthe presence of well-known reformer catalysts containing nickel, cobalt,etc., promoted with magnesia, alumina, thoria and similar oxides, andsupported, if desired, on an inert base. The temperatures usuallyemployed are in the order of 1200-1400 F. and higher, and as a result ofthe reaction, H2, CO2 and C are obtained and, because of the hightemperature required to promote the reformation of methane, the ratio ofCO to CO2 in the product is high. In order to increase the yield of H2the reaction mixture is passed from the first stage to a second stage,which is operated at a much lower temperature,

in the neighborhood of about 800 F., and further quantities of steam aregenerally added to the gas. In this stage the carbon monoxide producedin the first stage is converted to carbon dioxide and additionalhydrogen in accordance with the water-gas shift reaction, and the secondstage is catalyzed by CO converter catalysts rich in iron oxide, admixedwith one or more of a variety of other metals or metal oxides. Sincecarbon monoxide conversion is limited by watergas shift equilibriumconsiderations, complete conversion of CO to CO2 is not realized.

Where hydrogen of a higher purity is required, the gas from thewater-gas shift reaction, after condensation of the excess water vaporand removal of the carbon dioxide, can be mixed with additional steam,heated to an elevated temperature and passed over another quantity ofcarbon monoxide conversion catalyst. This results in conversion of amajor portion of the residual carbon monoxide to carbon dioxide with theformation of additional hydrogen. If an even higher purity is requiredthe above procedure 2 can again be repeated after removal of the carbondioxide. The carbon monoxide can also be removed by absorption in aselective solvent, usually a solution of a copper salt, or it may behydrogenated to methane by passing the gas over a suitable catalyst.These additional steps would be simplified or made unnecessary if anactive catalyst were available which would promote the water-gas shiftreaction at a sufficiently low temperature.

One of the problems associated with carbon monoxide converter catalystsis their tendency to be fragmented and crushed under the reactionconditions, resulting in formation of fines, dustiness, and otherundesirable conditions, preventing this use under conditions of high gasvelocities, thus seriously limiting plant throughputs.

The first steps in one customary process of preparingconverter catalystconsist in impregnating iron oxide with a solution of a promoter, suchas chromic nitrate to form a slurry, which is then heated in a furnaceto drive 01f the water and to decompose the nitrates to oxides. Theoxides are then wetted with waterand extruded to the desired size. Thepellets so formed are then dried in a steam heated oven at about 300-310 F. to remove the moisture.

Converter catalysts prepared by the general process as outlined abovepossess a rather-low crushing resistance or side strength. By sidestrength reference is had to the testing method wherein'a pellet is laidon its side and the force necessary to crush and disintegrate the latteris determined in a conventional catalyst crushing machine. The driedpellets are found to have an average side strength of about 10 pounds,which is far too low for the catalyst to holdup under plant operatingconditions. To be employable on a commercial scale, this side strengthmust be increased up to at least 45 pounds for a pellet, and the higherstrength the better.

Neither catalytic activity alone nor physical composition of a catalystper se is a criterion of the suitability of a given substance or mixturefor use in the process. The choice of catalyst is governed by a varietyof factors of which catalytic activity is but one of importance, and therobustness of the material and economy of its preparation are of equalsignificance, as in the method of preparation of the catalyst. Thus, ina recent article by Christain and Boyd in Chemical Engineering, May1949, it was shown that though three of the five principal commercialcarbon monoxide conversion catalysts were of comparable composition.commercial practice has shown that their performance is different andthis diiference is attributed to the method of manufacture rather thanto any small dinerence in composition. The authors conclude thatthemethod of manufacture has a large influence on the activity of acatalyst, and is a major factor in determining the physical properties.Thus, size of catalyst particles must be such that the allowancepressure drop across the catalyst bed will not be exceeded underoperating conditions. However, some catalysts, when subjected torelatively high space velocities, are partially reduced to a dust whichaccumulates in the catalyst bed. Channeling of the gas flow, increasedpressure drop and unsatisfactory conversion of carbon monoxide resultfrom dusting, and eventually the operation has to be interrupted and thecatalyst either removed and screened, or discarded and replaced with thefresh catalyst.

Prior to the present invention, it has been found that the physicalresistance to disintegration, as measured in terms of side strength,could be substantially increased by subjecting the dried catalyst,prepared by impregnating iron oxide with a solution of chromic nitrateto form a slurry which is then heated in a furnace to drive off waterand decompose nitrates into oxides, to a roasting process in thepresence of a stream of air. The effect of this roasting was not only toincrease the strength of the catalyst but also to decompose completely,salts, such as nitrates. In general, the heat treatment was carried outfor aboutS hours at about 1050 F. The table below indicates the effectof this heat treatment with air at increasing temperature levels, allcarried out in periods of 8 hours.

Percent of C in Exit Temperature of Treatment in Air, F. G

Thus, at those activation temperatures resulting in highly activecatalyst, the former has a low side strength. When side strength israised, activity falls.

When CO is present in the hydrogen leaving the converter and thehydrogen is employed in chydrogenation processes, the 00 will beconverted to CH4. Because of the enormous quantities of hydrogen thusmanufactured and thus employed, the methane concentration will rapidlybuild up to such a high degree that, in the hydrogenation process, someof the gas will continually have to be bled oil that ordinarily would berecycled, and thus relatively small differences in CO content of thehydrogen gas feed to a hydrogenation process can have a very appreciableeffect on the operating cost. For this reason, the importance ofreducing the CO content even by a little as 0.143% of the totalconverter eiiluent cannot be overemphasized.

It is thus apparent that in accordance with prior art practice, acompromise must be efiected between the higher catalyst treatingtemperatures favoring increased catalyst strength, and the lowertemperatures favoring increased activity. In general, a temperature of1050 F. is employed, resulting in an average side strength of about 45and an activity, as expressed in percent CO in efiluent from theconverter, of about 1.8 to 2.0.

Besides, the problem of activity and durability of the convertercatalyst prepared as indicated above, the method of preparation of sucha'catalyst, which consists of about 93% F8203 and 7% ClaOs, is tediousand lengthy, requiring, as it does,

4 seven distinct steps, namely (1) impregnation,

(2) drying and decomposition of nitrates, (3) wetting for extrusion, (a)extrusion, (5) drying, (6) activating, and (7) screening. Furthermore,the decomposition of nitrates poses the problem of disposing of theobjectionable oxides of nitrogen, for which there must be made suitableprovisions. It is highly desirable, therefore, to .produce a COconverter catalyst which may be prepared in a more simple fashion, andwhich will be more stable to heat and show higher conversion levels anddurability characteristics than the conventional catalyst prepared, asoutlined above, from iron oxide and chromic nitrate.

It is, therefore, the principal object of the present invention toprepare an exceptionally active carbon monoxide conversion catalyst in aform having a much greater physical strength and showing a greatlydecreased tendency towards dusting than conventionally preparedcatalysts.

It is also an object of the present invention to prepare a betterconversion catalyst for CO comprising chromium and iron by a processconsiderably more simplified than conventional processes.

A further object of the invention is to prepare a carbon monoxideconversion catalyst which is more stable to heat and to water than thoseprepared hitherto.

Other and further objects of the invention will appear from thedescription hereinafter.

It has now been found that an exceptionally rugged and active convertercatalyst comprising iron and chromium may be preparedin a considerablymore simplified manner than possible hitherto, by employing chromiumtrioxide instead of chromium nitrate and by a process of operationdescribed more fully below. The conventional process starts with amixture of chromium nitrate and red iron oxide; the improved processemploys CI'Os, instead of chromium nitrate.

,Since the amount of Water required to dissolve the necessary amount of(3103 is smaller than the amount required to dissolve the necessaryamount of Cr(NOs)3 and since the amount of water used to dissolve theC103 is adjusted to exactly the amount required for extrusion, theprocess is simplified by the elimination of a Wetting and drying step.The solubility of Cr(NOa)3 in water is substantially less than that ofchromic acid, in

terms of total amount of chromium that may be dissolved.

Furthermore, in accordance with the process of the present invention thedecomposition of the nitrates, with accompanying problems of disposingof the objectionable oxides of nitrogen, is eliminated. There isobtained a catalyst consisting essentially of oxides of iron andchromium which is substantially identical in composition with thecatalyst prepared by conventional means, but which, because of itsmanner oi preparation, is considerably superior in activity anddurability to the conventionally prepared converter catalyst. In orderto illustrate the invention, below are set forth specific examplesdescribing the improvement in catalyst preparation. Example I describesthe conventional process for preparing a catalyst consisting essentiallyof 93 FezOs .and 7% CrzOs, while Example II describes a process forpreparing the catalyst of the present invention which, though havingsubstantially the same chemical composition, has far superiorproperties.

EXAMPLE I A batch or conversion catalyst was prepared by impregnating ina mixer about 300 pounds of red iron oxide (F8203) with 11.5 gallons ofchromic nitrate (50% solution). It takes this large amount of water todissolve the chromic nitrate. However, this mixture is too wet toextrude and too dry to feed readily to the Herreshofi furnace.Accordingly, five gallons of water were added to make a slurry, whichwas conveyed to a Herreshofi furnace, wherein it was heated at about 550F. In this stage, substantially all of the water is driven ofi and mostof the nitrates are decomposed to oxides. However, in this drying step,about 34% of the material is lost out of the furnace stacks as a linepowder.

The resulting mixture consisting of about 93% iron oxide and 7% chromiumoxide is then wetted with water in a mixer to iacilita-te extrusion,approximately 7 gallons water/300 pounds of oxides being required. Afterextrusion and pelleting, the pellets are dried with steam at about 300F; to remove most of the moisture and to prevent cracking of the pelletson subsequent heating'at higher temperatures. The dried pellets are thenactivated in an atmosphere of air in an electric furnace for about eighthours at 1050" F. These are the time and temperatures required todecompose thoroughly all the nitrates and to make the pelletssufiiciently strong, imparting an average side strength of about 50pounds. The pellets after activation were screened to remove fines.

EXAMPLE 11 As an "example of preparing catalyst in accordance with thepresent invention, about 150 pounds of red iron oxide (F8203), 30 poundschromic acid (CrOs) and 5.5 gallons of water were made into a paste in amixer and, after thorough mixing, another 150 pounds of iron oxide wereblended in and mixed well. The resulting material was next extruded tothe desired size, to pellets of about 1='%"-%" being the usual plantsize. It may be noted here that, for the same consistency, the chromicacid containing material was more readily extruded than the conventionaltype material, and thus, a drier matepellets were dried and activated inan electric furnace for 8 hours at 1100" F., which tempera-' ture wasfound to give the best activity and strength, and at which temperatureCrOa is converted into CrzOs. After activation the pellets were screenedtoremove fines. The elimination of the drying step before extrusioneliminated dust losses as well.

Comparing the two processes, therefore, it can be seen that inaccordance with the present invention, three steps are eliminated.

Old Process New Process (1) Impregnation in Mixer (1) Impregnation inMixer.

Drying in Furance at 550 F (2) Extrusion.

(3) Wetting for Extrusion in Mixer (3) Active on at 1,l00 F. (4)Extrusion (4) Screening. (5) Drying at 300 F (6) Activation at 1,050 (7)Screening A comparison of activity 0d. the two types of catalyst isgiven in the table below in Example EXAMPLE III Catalysts prepared byboth processes were tested for activity in a carbon monoxide conversionunit. As feed gas, C0, C02. H2, and steam, representing the efiluentfrom a prior methane reforming unit, were employed.

Comparison of activity ofwater-gas shiftcdta Zyst: 7% CrzOs; 93% FezOaCatalyst Mfg. Process Old New Vol. Catalyst "cc. (4-8 mesh) Wt. Catalyst.gran1s cou q carom awmoaororal cow-ms Feed Gas Composition:

Percent 00, Vol. Percent Percent CO2, Vol. Percent Percent H1, Vol.Percent Exit Gas:

Percent 00 Percent CO Percent Hi EXAMPLE IV Percent C 0 in EffluentSample March 3- March ll-l'i March 18-20 At the temperature level of800-900 F., which is most desirable for operating the water gas shift,'or carbon monoxide conversion reaction, the equilibrium value in termsof unconverted CO is about 1.2%.

With the conventional commercial catalysts under commercial operatingconditions, the CO in the efliuent is in the neighborhood of 2.0-2.5%.

Examples I11 and IV clearly indicate the superior activity of theconversion catalyst prepared in accordance with the present invention.Beside the examples cited, numerous other examples may be given.

As already indicated, it is necessary tmh'eat converter catalyst torelatively high temperatm'esijm ordert'to. obtain. the necessaryphysical; strength for commercial use; otherwise, the par:- ticles will.fall .apartin the plant, with; inefficient operation. resulting.However, ithas been found that: heating the catalyst: may impair;activity anchasalready shown with conventional. catalyst, a compromisemust be reached between activity and physicalstrength; that is, above acertain temperature level-,ahont 1000l100 R, increasing the temperatureof activation in! creases. the strength but markedly decreases theactivity. With the new catalyst, however, activation may be carried outat considerably higher temperatures without accompanying loss in activity. Thusin the fo'llcwingtable' it may beseen that catalyst from theimproved process has not only a greater heat .stabilityfbut also greaterphysical strength than that prepared .by the conventional process.

20, Conventional Cat- New Catalyst ahst.

3353: Pellet Pellet Strength 65 3 Strength g @2333? Product 1 2235?Product These data shownot only the. greater heat stability of the newcatalyst; but'also'its greater strength.

Thus tosummarize, "the advantages of a 'Ci2O3F2O3 catalyst prepared. inaccordance with the present invention'over a: conventionally pre-- paredcatalyst are as follows:

(1) Better activity;

(2) Better physical-strength? (3) Catalyst does not'break down when-wetwith water; sometimes in the operation of plants for carrying out thewater-gas shift reaction, the catalyst may become'wettedwith water, andit is importantr that, the particlesibe able to with stand this wetting.treatment without physical disintegration. It has been demonstrated thatthe improved catalyst does. not lose physical strength when wetted,while theunimproved is seriously weakened.

i Better heat stability.

7 (5) Les expensive catalyst in terms of chemicals, labor andtransportation.

(5) Better gas for hydrogenation processes, as it contains less CC).

(7) By eliminating the conventional wetting anddrying steps. 3.5-4%ilosses of materiai :are

prevented. I

(8). Since-the: mixture produced in the modi- 'ned ..procedure is'rnotas wet at the time of extrusion as that produced by older processes, theextruded catalyst from the modified. process can be sent at once to theactivation furnace with outgoing. througha preliminary drying step.

- Numerous modifications of the invention are readily apparent. to thoseskilled in the art. Thus, small variationsin the ratios of thecomponentsofpthe final catalyst, or. in the ratio of chromic acid andiron oxide fed initially, are

:within: the scope-ofthe invention.

- This; m accordance-with the'present inventiomzthere-is obtained awater gas shift catalyst which, when employed in a carbon monoxide.conversion zoneoperated in the neighborhood of about 800 -900 F.,reduces the CO content of the eifluent from a. methane or natural gasreforming zone to substantially equilibrium proportions. This catalystcombines a high activity with an exceptional degree of ruggedness.

What is claimed is:

1. An improved process for converting a gaseous mixture comprising C0,C02, and H2 into a product. rich in hydrogen and low in carbon monoxidewhich-comprises passing said mixture and steam through a carbon monoxideconversion zone in they presence of a CO converter catalyst comprising.a major portion of Fe20: and a minor portion of CrzOa prepared byimpregnating iron oxide with an aqueous solution of chromic acid,extruding and pelleting the resulting mixture and activating the pelletsat temperatures in the range of 1000-l300 2. The process of claim 1wherein said gaseous mixture passed-to the conversion zone compriseseiiluent from a methane reforming zone.

3. An improved process forconverting a gaseous. mixture comprising '60,CO2 and H2 into a product rich in hydrogen and low in carbonmonoxideswhichcomprises passing said mixture and-steam through a carbonmonoxide conversion zone in the presence ofa CO converter catalystcomprising about 93% FezOs and about 7% CIzOg prepared byimpregnating'iron oxide with an aqueous solution of chromic acid,extruding and p'elleting the resulting mixture and activating thepellets. at temperatures in the range of aboutl000to about 1300 F.

4'. In: the. process of producing a catalyst comrisingta majorproportion of F6203 and a minor proportion of CrzOi adaptedto promotethe water gas shift reaction, the improvement which comprisesimpregnating iron oxide with an aqueous solution. of chromic acid toform a paste-like mixture, extruding the resulting mixture, activatingthe. extruded material at elevated temperaturesin the range of about1000 to 1300 F., and recovering a carbon monoxideconverter catalystcomprising a major proportion of R203 and-a minor proportion of Crz0a-..

5. The process. of claim vievvheisein said-extraded material ispelleted-s 6. Inv the process of. producing awater gas shift catalystconsisting essentially oi ferric oxide and .chromic oxide, the;improvement which comprisesiorming a slurry of ferric oxide, chromicacidgeand water in a mixing zone, adding a furtherzquantity of ferricoxide to said slurry taproduce a material suitable for extruding,extruding the latter, pelleting said extruded material, activating saidpellets in the presence of air ata temperature of about l0501250 F. fora period up to about 8 hours,v and obtaining a highly active catalystof.exceptional rug edness com- .prising amajor proportion of ferric oxideand a impregnating iron oxide with, an aqueoussolntion of chromic acidvto form a paste-like mixture, extruding the resulting mixture andactivating. the extruded material at temperature of about 1 090 tovabout 1350 8. The processof producing animproved water gas shiftcatalyst consisting. essentially of'about 93%-F6203 and of about 7% C:which comprises forming a slurry of ferric oxide, chromic 10. Thecatalyst of claim 9 wherein said cataacid and water in a mixing zone,adding a furlyst consists essentially of 93% F6203 and 7% ther quantityof ferric oxide to said slurry to pro- Cr203. duce a material suitablefor extruding, extruding 11, The catalyst of claim 9 wherein saidcatathe latter, pelleting said extruded material, acti- 5 lyst has acrushing strength greater than 90 vating said pellets in the presence ofair at a tempounds. perature of about 1050 to about 1250 F. for a OTTISW. MOAK. period of up to about 8 hours and obtaining a WILLIAM E.SPICER. highly active catalyst of exceptional ruggedness.

9. An improved carbon monoxide conversion 10 REFERENCES CITED catalystadapted to promote the water gas shif The following references are ofrecord in the reaction comprising essentially a major portion file fthis patent; of ferric oxide and a minor portion of chromic oxide, saidcatalyst having a pellet crushing UNITED STATES PATENTS strength of atleast 80 pounds at an activity level 15 Number Name Date correspondingto substantially equilibrium car- 1,107,581 Brownlee et al Aug. 18, 1914bon monoxide conversion in the temperature 1,330,772 Bosch et a1 Feb.10, 1920 range of about 800-900 F., said catalyst being 1,934,795 FrazerNov. 14, 1933 prepared by impregnating iron oxide with an 2,408,140Gutzeit Sept. 24, 1946 aqueous solution of chromic acid to form a paste-:0 2,461,147 Davies Feb. 8, 1949 like mixture, extruding the resultingmixture and activating the extruded material at a temperature of fromabout l000 to about 1300 F.

1. AN IMPROVED PROCESS FOR CONVERTING A GASEOUS MIXTURE COMPRISING CO,CO2, AND H2 INTO A PRODUCT RICH IN HYDROGEN AND LOW IN CARBON MONOXIDEWHICH COMPRISES PASSING SAID MIXTURE AND STEAM THROUGH A CARBON MONOXIDECONVERSION ZONE IN THE PRESENCE OF A CO CONVERTER CATALYST COMPRISING AMAJOR PORTION OF FE2O3 AND A MINOR PORTION OF CR203 PREPARED BYIMPREGNATING IRON OXIDE WITH AN AQUEOUS SOLUTION OF CHROMIC ACID,EXTRUDING AND PELLETING THE RESULTING MIXTURE AND ACTIVATING THE PELLETSAT TEMPERATURE IN THE RANGE OF 1000*-1300* F.